Power-Supply Considerations
Overview
The MAX349/MAX350 construction is typical of most
CMOS analog switches. It has three supply pins: V+, V-
and GND. V+ and V- are used to drive the internal
CMOS switches, and they set the limits of the analog
voltage on any switch. Reverse ESD-protection diodes
are internally connected between each analog signal
pin and both V+ and V-. If any analog signal exceeds
V+ or V-, one of these diodes will conduct. During normal
operation, these (and other) reverse-biased ESD diodes
leak, forming the only current drawn from V+ or V-.
Virtually all the analog leakage current is through the
ESD diodes. Although the ESD diodes on a given sig-
nal pin are identical, and therefore fairly well balanced,
they are reverse biased differently. Each is biased by
either V+ or V- and the analog signal. This means their
leakages vary as the signal varies. The
difference
in the
two diode leakages to the V+ and V- pins constitutes
the analog signal-path leakage current. All analog leak-
age current flows to the supply terminals, not to the
other switch terminal. This is why both sides of a given
switch can show leakage currents of either the same or
opposite polarity.
There is no connection between the analog signal
paths and GND.
V+ and GND power the internal logic and logic-level
translators, and set both the input and output logic lim-
its. The logic-level translators convert the logic levels to
switched V+ and V- signals to drive the analog signal
gates. This drive signal is the only connection between
the logic supplies (and signals) and the analog sup-
plies. V+ and V- have ESD-protection diodes to GND.
The logic-level inputs and output have ESD protection
to V+ and to GND.
The logic-level thresholds are CMOS and TTL compati-
ble when V+ is +5V. As V+ rises, the threshold increases
slightly. Therefore, when V+ reaches +12V, the threshold
is about 3.1V; above the TTL-guaranteed high-level mini-
mum of 2.8V, but still compatible with CMOS outputs.
MAX349/MAX350
Serially Controlled, Low-Voltage,
8-Channel/Dual 4-Channel Multiplexers
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